adl5331 Analog Devices, Inc., adl5331 Datasheet - Page 11

• Manufacturer Part Number: adl5331
• Description: 1 Mhz To 1.2 Ghz Vga With 30 Db Gain Control Range
• Manufacturer: Analog Devices, Inc.

The device can be driven single-ended with similar perfor-
mance, as shown in Figure 17. The single-ended input interface
can be implemented by driving one of the input terminals and
terminating the unused input to ground. To achieve the optimal
performance, the output must remain balanced. In the case of
Figure 17, a transformer balun is used at the output.
GAIN CONTROL INPUT
When the VGA is enabled, the voltage applied to the GAIN pin
sets the gain. The input impedance of the GAIN pin is 1 MΩ.
The gain control voltage range is between 0.1 V and 1.4 V,
which corresponds to a typical gain range between −15 dB and
+15 dB.
The 1 dB input compression point remains constant at 3 dBm
through the majority of the gain control range, as shown in
Figure 7 through Figure 9. The output compression point
increases decibel for decibel with increasing gain setting. The
noise floor is constant up to V
The bandwidth on the gain control pin is approximately 3 MHz.
Figure 10 shows the response time of a pulse on the V
Although the ADL5331 provides accurate gain control, precise
regulation of output power can be achieved with an automatic
gain control (AGC) loop. Figure 18 shows the ADL5331 in an
AGC loop. The addition of a log amp or a TruPwr™ detector
(such as the AD8362) allows the AGC to have improved
temperature stability over a wide output power control range.
RFIN
Figure 17. Single-Ended Drive with Balanced Output
10nF
10nF
INHI
INLO
ADL5331
RF VGA
OPLO
GAIN
OPHI
120nH
= 1 V where it begins to rise.
5V
120nH
10nF
10nF
ETC1-1-13
RFOUT
GAIN
pin.
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Note that the ADL5331, because of its positive gain slope, in
an AGC application requires a detector with a negative V
RF
Figure 19 has a negative slope. The AD8362 rms detector,
however, has a positive slope. Extra circuitry is necessary to
compensate for this.
To operate the ADL5331 in an AGC loop, a sample of the
output RF must be fed back to the detector (typically using
a directional coupler and additional attenuation). A setpoint
voltage is applied to the VSET input of the detector while VOUT
is connected to the GAIN pin of the ADL5331. Based on the
detector’s defined linear-in-dB relationship between VOUT
and the RFIN signal, the detector adjusts the voltage on the
GAIN pin (the detector’s VOUT pin is an error amplifier
output) until the level at the RF input corresponds to the applied
setpoint voltage. The V
in the correct balance between the input signal level at the
detector and the setpoint voltage.
The detector’s error amplifier uses CLPF, a ground-referenced
capacitor pin, to integrate the error signal (in the form of a
current). A capacitor must be connected to CLPF to set the
loop bandwidth and to ensure loop stability.
RFIN
IN
slope. As an example, the
DAC
INHI
INLO
VSET
VPOS
Figure 18. ADL5331 in AGC Loop
LOG AMP OR
5V
DETECTOR
ADL5331
TruPwr
VOUT
GAIN
CLPF
GAIN
COMM
setting settles to a value that results
OPLO
OPHI
RFIN
AD8319
5V
in the example in
ADL5331
DIRECTIONAL
COUPLER
ATTENUATOR
OUT
/